Just say no to series capacitors because you don't have to go there, plenty of 450V rated parts are available. If you have shape constraints and a single part doesn't meet your energy storage requirement and can't fit a larger single case, use two 450V parts in parallel instead.
Series capacitors, even with some high-value balancing resistors, require a bit of extra voltage derating so 2*250V is not actually 500V, a single 450V rated part is better.
Key to very long lifetime is,
1) using well known brands,
2) parts rated to 105 degC or more, and rated for some 5000 hours or more,
3) derating voltage and ripple current rating, maybe to 80% and 50%, respectively, if you can do that, that is,
4) derating temperature rating, i.e., make sure the local ambient doesn't go nowhere near the rated 105 degC.